1. Field of the Invention
The invention in general relates to the formation of sonar beams, and particularly to an arrangement utilizing CCD's and a SAW device for forming multiple beams in the near field for focusing purposes.
2. Description of the Prior Art
The chirp Z-transform is a Fourier transform technique utilized in the sonar field for forming multiple beams in the far field. The technique is used in conjunction with an elongated multi-element transducer array wherein the output signals from the elements are provided to a charge coupled device (CCD). The CCD is read out at a certain rate and is mixed with a frequency chirp signal, that is, a signal whose frequency varies linearly with time. The resultant signal is then "de-chirped" in a convolver consisting of a surface acoustic wave (SAW) device, the output signal of which is detected, the detected signal constituting the multiple beam signals.
The apparatus is utilized for forming multiple beams in the far field. For many applications, however, it would be desirable to use such apparatus to focus in the near field for high resolution work. The present invention accomplishes this objective.